refactor: rename frontend/ to web-player/ and format with prettier

Updates the compose build context, the Dockerfile header, the package
name and the docs. The compose service and container keep the name
frontend/musify-frontend, since nginx routes to http://frontend:3000.

The source files also carry the first prettier pass, so the rename and
the reformat land together for them.

web-player/.vscode/extensions.json

@@ -0,0 +1,3 @@
+{
+	"recommendations": ["svelte.svelte-vscode", "bradlc.vscode-tailwindcss"]
+}

web-player/.vscode/settings.json

@@ -0,0 +1,5 @@
+{
+	"files.associations": {
+		"*.css": "tailwindcss"
+	}
+}